Need some opinions about hard drive configs

So I'm thinking about redoing the hard drive configuration on my Dell server and I am a bit undecided about how I want to do it. Right now it is as follows:Machine: Dell PowerEdge 2970Current Drive Config: OS on a single 146GB 15K SAS ; Mass storage on 4x 72GB 15K SAS in raid 5 (stripe with hot spare)

This config has my OS on a single drive and all of my data including game servers on a raid 5 for data integrity. Here is what I am thinking about doing:

I want to have a mirrored array set up so that I can store all of my files without worrying about a disk failure. What I'm not sure about is should I buy a second 300GB 15K SAS drive for the mirror? (I already have one on hand) or Should I just buy 2 larger SATA Drives? 300GB should be enough space for me, but they are around $250 each. SATA drives are much cheaper, but I'm worried about how they might play with each other. Basically the mass storage array will contain all of my work files as well as backup copies of all the server and game files on my steam account (for fast reinstall). So speed is not an issue on that array, instead compatability is. What do you guys think?